Known in the art is an internal combustion engine arranging an NOX selective reducing catalyst in an engine exhaust passage, feeding aqueous urea into the NOX selective reducing catalyst, and using the ammonia generated from the aqueous urea to selectively reduce the NOX contained in the exhaust gas, which internal combustion engine recovering moisture from the exhaust gas and mixing solid urea with the recovered moisture to thereby generate aqueous urea to be fed to the NOX selective reducing catalyst (for example, see Japanese Patent Publication (A) No. 2002-510005). If using solid urea as urea generating aqueous urea, there is the advantage that it is possible to lower the frequency at which urea is replenished.
However, there is the problem that the control of the urea concentration of the aqueous urea is difficult when using this kind of solid urea.